The cytotoxicity for cancer cells of fucoidans isolated from 6 brown seaweed species was investigated. All fucoidans showed cytotoxic activity against the RD (Rhabdomyosarcoma) and HepG2 (hepatocellular carcinoma) cells. From small-angle X-ray scattering (SAXS) measurements, fucoidans have a branched structure, as indicated by the R-gc values ranging from 0.78 to 1.51nm. Our results supported the branched architecture with many sulfate groups reveals bioactivity of fucoidan but the relationship between the length of the branch and the level of bioactivity was not clear.